It is known that it is necessary to have a certain degree of roughness on each cylinder wall of an engine to retain oil used as a lubricant between the cylinder wall and the piston which is slidingly engaged within the respective cylinder. In practice, the surface of the cylinder wall comprises a number of peaks and troughs and the oil is stored within the troughs. It is further well known in the manufacture of internal combustion engines to use a honing process to produce a desired surface finish on the wall of each cylinder of the engine to provide such an oil retention surface.
In recent years there has been an increased demand for the use of emission control devices on all engines used for automotive vehicles. In the case of spark ignition engines the use of NOx traps is becoming more common and in the case of diesel engines the use of a particulate filter and in some cases a NOx trap is becoming increasingly a requirement.
There is a requirement to periodically regenerate any particulate filter or NOx trap fitted to such an engine to maintain the efficiency of the particulate filter or NOx trap.
In the case of diesel engines, regeneration can be brought about by late or post-injection of fuel into each cylinder. U.S. Patent application 2003/0056498-A1 provides an example of a method and device for regenerating a particulate filter used with a diesel engine.
It is also a requirement to regenerate an emission device of a spark ignited engine and in so doing it is a possibility that some unburned fuel may contact the cylinder walls and may result in the fuel contaminating the lubrication oil.
There are other situations in which unburned fuel may collect on the cylinder walls such as pilot injection or when multiple injections are used.
In any of these cases it is a problem that some of the fuel injected into each cylinder for regeneration purposes remains within the troughs in the surface of each cylinder wall and is subsequently transferred past the piston rings to the oil used to lubricate the engine.
The addition of this fuel to the oil can produce problems and, in particular, can lead to a reduction in oil viscosity with a consequential reduction in oil lubrication performance.
It is an object of this invention to provide an engine with less susceptibility to fuel to oil transfer and to provide a method for manufacturing such an engine.